Molecular sieves of the crystalline aluminosilicate zeolite type are well known in the art and now comprise over 150 species of both naturally occurring and synthetic compositions. In general, the crystalline zeolites are formed from corner-sharing AlO.sub.2 and SiO.sub.2 tetrahedra and are characterized by having pore openings of uniform dimensions, having a significant ion-exchange capacity and being capable of reversible desorbing an adsorbed phase which is dispersed throughout the internal voids of the crystal without significantly displacing any atoms which make up the permanent crystal structure.
One of these synthetic zeolites is EMC-2, which is composed of sodalite cages arranged in a hexagonal array and has been designated as having the EMT framework structure by the International Zeolite Association (See, Atlas of Zeolite Structure Types, W. M. Meier and D. H. Olson, 3d Edition (1992), Butterworth-Heinemann). The EMC-2 zeolite is synthesized using the organic directing agent 18-crown-6-ether. Synthesis of this zeolite as well as structure characterization has been reported in the following: F. Delprato et al., Zeolites, 10, 546-552 (1990); Ch. Baerlocher, Acta Cryst., A46, C177 (1990).
As synthesized, the EMC-2 zeolite has a SiO.sub.2 /Al.sub.2 O.sub.3 ratio from about 6.5 to about 10.0. Applicants have successfully treated EMC-2 zeolites with a fluorosilicate salt to simultaneously remove aluminum atoms and insert silicon into the framework, thereby giving a SiO.sub.2 /Al.sub.2 O.sub.3 ratio of about 6.6 to about 11.2. The zeolite obtained from this process has been identified as LZ-281, has retained the EMT framework structure and at least 50% (preferably 90%) of the relative X-ray crystallinity of the starting EMC-2 zeolite. It is also observed that the LZ-281 zeolite has increased acidity and cracking activity.
U.S. Pat. No. 5,425,934 discloses dealuminating an EMT zeolite by contacting the parent zeolite with a 1M methanolic HCl solution. However, the '934 reference does not mention adding silicon back into the sites vacated by the aluminum. That is, the LZ-281 of the invention will have less defect sites than the EMT zeolite of the '934 reference.
U.S. Pat. No. 4,711,770 discloses a process for treating a zeolite with a fluorosilicate salt. The '770 patent discloses treating a zeolite Y with a fluorosilicate salt, thereby removing aluminum atoms from the framework and inserting silicon. However, the '770 patent does not disclose treating an EMC-2 zeolite.